Over the past few years, I've had some fantastic success with writing query letters, primarily due to a handful of articles and how-to's that I can no longer find. The new articles I see in all of the writing magazines are always vague and always have some sort of "niche" idea for how to write a query letter. I'm here to tell you that you don't need a niche to hook agents in. You just need a formula and a little creativity. I'm assuming you know how to address a letter, so we'll get right to the meat of it ...
Option 1: The "Hook" approach
First Paragraph: The hook. Take the most interesting part of your story (fiction or non-fiction) and use it to hook the agent right off the bat. Imagine this first paragraph as the "Back of the DVD case." You have 3-4 sentences to get someone to "buy" your story.
Second Paragraph: Elaborate. Explain what the book is, and compare it with other books that are similar and have sold a lot of copies. Explain what makes your book unique.
Third Paragraph: A little about yourself. Cite your experience in writing, how you can help sell the book, any other awards/contests/publications you've had.
Done. That's it. End it with "The full manuscript is ready to be shown, if you're interested. Thank you for your time and I look forward to hearing from you."
Option 2: The more formal approach
First Paragraph: "[Title] is a finished [fiction or non-fiction] manuscript that I would like to show you. It is [Number] words and [add any researched information, relevance to agent's interests].
Second Paragraph: Short synopsis of the book. Try to outline as much of the book in as few sentences as possible, and remember to work to bring out the main conflict of the story (if it's fiction) or the main point (if it's non-fiction).
Third Paragraph: Compare the manuscrip to published books like it that have sold well, and explain what makes your story (or non-fiction book) unique.
Done. End the letter in the same way you would with option 1.
And that's it! It's simple, no longer than a page, and aimed at catching the agent's attention either with a good "hook" or a very professional-looking letter.